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Programming Distributed Systems with YAMI4

10.2.4 Interfaces And Messages

Interface is a named group of messages and is an analogue of an interface in object-oriented design.

Messages have names and can have associated payload for both request and reply interactions. A simple message can be defined without any payload:

   interface Device is
      message Start;
      message Stop;
      --  ...
   end Device;

Above, the Start and Stop message does not have any payload, but both messages indicate two-way interactions, which means that their processing can be confirmed by receiver.

One-way messages are used for interactions without confirmation:

   interface Device is
      --  ...
      oneway message Keep_Alive;
   end Device;

Messages can have payload for either request or reply direction, or both. The payload, if present, is always a single value of the user-defined type with an indication of direction:

   type Time is
      Timestamp : Long_Long;
   end Time;
   interface Clock is
      message Set_Time (T : in Time);
      message Get_Time (T : out Time);
   end Clock;

Above, Set_Time is a message that will contain the Time record as a parameter and no payload in reply, whereas Get_Time will have no parameters in the request, but will deliver the Time value as a reply.

Of course, data can be transmitted in both directions as well, as can be seen in the complete package definition of the calculator example:

package Calculator is

   type Operands is
      A : Integer;
      B : Integer;
   end Operands;

   type Results is
      Sum : Integer;
      Difference : Integer;
      Product : Integer;
      Ratio : optional Integer; -- does not exist if B = 0
   end Results;

   interface Operations is
      --  message with data tramsmitted in both directions:
      message Calculate (Op : in Operands; Res : out Results);
   end Operations;

end Calculator;